DISCUSSION OF SPECIES AND THEIR DISTRIBUTION. 5 



The color is dark, except upon the l>elly, which is grayish. The second dorsal is darker 

 across the middle infronl and towards the margin behind, while the caudal is blackish 

 throughout. 



MEASUREMENTS. 



Inches. 



Total length 11 



Snout tn eve 1 



Si km it in lirst branchial aperture 1 » 7 , T 



Snout In seven lb branchial aperture '2-f,- : 



Sin i ut to commencement of first dorsal 5| 



Snout to cnil of lirst dorsal 6| 



Snout to commencement of second dorsal 7^ 



Snout to cud of second dorsal 10 



Snout to end of caudal 11 



Snout to commencement of caudal below 9}£ 



Snout to commencement of caudal above 10 



Snout to :i n us 7$ 



This hitherto undescribed form is evidently most closely related to the typical species 

 of the genus Petromyzon, but differs by the obsolescence of the armature of the suproral 

 and iufroral lamina-, while differences of proportion characterize the species; it is scarcely 

 generically distinct from Petromyzon, but may be distinguished as a subgeneric type under 

 the name Bathymyzon with the following characters: 



BATHYMYZON. 



Petromyzontinm with the suproral lamina contracted, its two converging teeth almost 

 completely fused together and only evident at the summit of the combined mass, infroral 

 lamina crescentiforin and spout-like at the middle, and with the denticles obsolete, discope- 

 ripheral teeth numerous and in obliquely-arched series of 4-7, declining downwards; the 

 innermost lateral teeth of the four rows diverging from the mouth, in each side bicuspid, 

 with the cusps approximated, and diminishing downwards rapidly; the lingual teeth 3, 

 pectinate, the anterior deeply impressed and sulcate backwards at the middle and the pos- 

 terior correspondingly curved backwards at their inner lateral angles; the anterior dorsal 

 tin distinct from posterior. (6ill, MS.) 



Class ELASMOBRANCHIL 



Lyriferous vertebrates with cartilaginous skeleton, and destitute of membrane or der- 

 mal bones; no cranial sutures. Body with vertical and paired fins, the posterior pair ab- 

 dominal; caudal tin with elongated upper lobe; gills attached by their outer edges to the 

 skin, with an intervening gill opening between each; no gill cover; skin naked or covered 

 with minute imbricated scales or hard plates, sometimes spinous; no air bladder; arterial 

 bulb with three series of valves; intestines with a spiral valve; optic nerves united, not 

 decussating, or only slightly so; ova few and large, fertilized, and sometimes developed 

 internally; embryo with external deciduous gills; males with intromittent organs attached 



to ventrals. 



KEY TO Sl'BCLASSES OF ELASMOBRANCHIL 



[, Gill openings, 5-7, slit dike; jaws detached from skull Selachii (Sharks and Rays) 



II. (Jill openings single, lour branchial clefts; jaw and palate attached to skull. . . Holocephali (Chima-rasi 



The class Elasmobranchii, intermediate between the true tishes and the Marsipobranchi 

 ates, is sparingly represented in the abyssal faunas. 



Subclass SELACHII. 



Elasmobrauchiates with body more or less cylindrical or depressed, with gill openings 

 slit like, five (sometimes six or seven) in number, sometimes lateral or inferior; jaws de- 

 tached from the skull; opercular and pelvic bones lacking. 



